Characterization of HetN, a protein involved in heterocyst differentiation in the cyanobacterium Anabaena sp. strain PCC 7120.

HetN, a putative ketoacyl reductase, is required for heterocyst pattern maintenance in the filamentous cyanobacterium Anabaena sp. strain PCC 7120. The hetN gene, when present on a multicopy plasmid, is able to suppress heterocyst differentiation. Little is known about the biochemical properties of HetN. In the present study, we found that HetN could hydrolyze ATP or GTP in vitro, and that this activity was dependent on the presence of magnesium in the reaction mixture. Mutations of the conserved active Ser142–Tyr155–Lys159 triad, predicted as necessary for the reductase activity of HetN, had only weak effect on the hydrolysis of ATP. The residue Lys159 is shown to be necessary for the heterocyst-suppressing activity of HetN, as the corresponding mutant allele present on a replicative plasmid failed to block heterocyst differentiation in contrast to the wild type. This result suggests that the reductase activity of HetN is involved in the HetN-mediated inhibition of heterocyst formation.